In the light of environmental problems, fuel cells have been attracting attention in recent years. Various kinds of research have been undertaken with a view to making them a practical reality. Some of the topics in this research have been methods for producing hydrogen and methods for supplying it. From the standpoint of safety during handling and the possibility of using existing infrastructure, various suggestions have been made regarding methods of reforming mineral oils and synthetic oils derived from fossil fuels, that is hydrocarbon fuel oils, into hydrogen.
For example, JP-A-2002-80868 and JP-A-2002-83626 disclose techniques which, by using in the fuel cells a fuel comprised of hydrocarbon compounds with specific composition and distillation characteristics, are intended not only to make it possible to obtain electrical energy at high outputs with a low rate of performance reduction, but also to satisfy various kinds of performance for use in fuel cells.
Also, in response to environmental issues, automotive exhaust gas regulations are becoming more widespread, and on account of these regulations there is demand for fuels that are sulphur free and have no soot emissions. In other words, making hydrocarbon fuels cleaner is an important issue. Meanwhile, there is demand for reducing the sulphur in fuels from the standpoint of making reformers more compact and of preventing catalyst degradation.
To produce fuels with a low sulphur content, it is beneficial in manufacturing terms to use fuel components that have a very low sulphur content. As examples of such fuel components, mention may be made of hydrocracked oils, lightened LGO (light gas oil), or GTL (Gas to Liquid) fuels produced by a Fischer-Tropsch reaction from synthesis gas (CO, H2). Of these, GTL fuels in particular have only minute sulphur and aromatics contents and so are regarded as promising for fuels of the future. They have also been considered in the aforementioned JP-A-2002-80868 and JP-A-2002-83626.
In fuel cell systems where hydrogen is obtained by reforming hydrocarbon fuel oil, since it is necessary to heat the fuel and reforming apparatus, if the temperature required to reform the fuel is low, the amount of heat will be reduced, and it may be said that the efficiency of the system will thus be increased, which is desirable.
As regards the heat required for reforming, methods of burning part of the fuel used for reforming may be considered, but if use is made of off-gas from the fuel cells, there are advantages such as being able to make the apparatus smaller, which is to be preferred. However, in this case, the amount of heat generated by the off-gas is preferably the amount of heat required to maintain the temperature of the reforming apparatus. This amount of heat generated by the off-gas is determined by the composition (in particular, the hydrogen and methane concentrations), but this composition is determined by the state of the reaction at the reforming apparatus outlet, that is by the outlet temperature. Therefore, in cases where the off-gas is a heat source for the reforming apparatus, the outlet temperature of the reforming apparatus is subject to a constraint in order to maintain the amount of off-gas heat generated. Consequently, in fuel cell systems where off-gas is used as a reforming apparatus heat source, it is more effective, in order to improve its efficiency, if the inlet temperature of the reforming apparatus is reduced.
Also, the lighter are the hydrocarbon fuel oils, that is, the higher is the proportion of hydrocarbons with a small number of carbons, the greater is the efficiency of conversion to hydrogen, and the more it is effective to make the fuels light in order to reduce the inlet temperature. But with light fuels there is a tendency for the flash point to decrease and there is a danger that safety will be compromised during handling. For this reason, from the standpoint of increasing production of hydrocarbon fuels, it may be said that it is preferable to make them as heavy as possible, that is to increase the proportion of hydrocarbons with a large number of carbons, but in the technology of the prior art no account has been taken of reducing the inlet temperature in the reforming apparatus. Indeed, there has been no research on making fuels heavier.
Furthermore, GTL fuels, which are regarded as promising for fuels of the future, are fuels in which paraffins are the main constituents with trace amounts of aromatics, olefins and naphthenes. Although there is a possibility that the distribution of their carbons and the composition ratio of isoparaffins and normal paraffins may affect the ease of reforming, there has been no research either that takes account of this point.